The present invention relates to a device for feeding a continuous web and cutting it into pieces, having automatic reel change of the reels from which the web is unwound.
In known devices of the said type the web is unwound from a reel and supplied to periodic cutting means which operate in cycles corresponding to the production of a cut piece. Such feed means normally include a pair of rollers, the said unwinding rollers, which unwind the web from the reel and supply it to the cutting rollers. A device of this type can be used for unwinding and cutting a web from which the outer wrappings or labels of cigarette packets are formed.
In this case the webs have designs printed on them, which designs must be constantly centred, that is to say must be located in a predetermined fixed position on each piece (label) separated by the cutting means. For the purpose of obtaining a correct and constant centring of the printed device on each individual cut piece it is necessary to perform a detection operation and a registration operation before the cutting operation. For this purpose detection means, for example of optical type, detect any possible phase displacement of the designs printed on each section of the web intended to constitute an individual cut piece, with respect to the correct centred position.
A signal emitted by the said detection means constitutes the control for a registration device which includes means operable to correct the quantity of web unwound by the unwinding rollers for the purpose of nullifying any possible phase displacement. In the prior art the correction, for the purpose of re-centralising the printed design, can be effected by web transport means completely independent from the unwinding rollers or by means which control the unwinding rollers by adjusting their speed of rotation. Such registration devices, forming part of the feeding and cutting devices described, as well as being complex and often bulky, do not have a satisfactory precision and speed of response for use at the very high speeds of modern packaging machines.
A further disadvantage of such known feed and cutting devices becomes apparent whenever it is necessary to change from the production of pieces of a given longitudinal dimension to pieces having a different longitudinal dimension. This problem is resolved in the prior art by a manual operation requiring replacement of the unwinding rollers with unwinding rollers of a different diameter. Devices of the type in question are, moreover, provided with an automatic reel change device for the reels from which the web is unwound; that is to say as well as a first reel there is normally provided a second reel from which a second web can be unwound to replace the first, and there are provided means for detecting when the web in one of the reels is running out. When such detection means are activated the web being cut into pieces is exchanged. Such detection means generally include photodetecting devices or switches for detecting when the radius of the reel reduces down to a lower limit.
Normally the leading end of the replacement web is maintained stationary in a convenient position, and is then made to advance in dependence on a signal generated by such detection means. In particular, in the device described in British Pat. No. 1 215 047, the end of the replacement web is maintained close to the cutter blades, and the web feed rolls for such replacement web are activated when the end of the first web is detected, for example by means of a photocell. This automatic reel change device has several disadvantages, among which are the fact that the first piece of replacement web is slightly shorter than the predetermined length, and moreover, by being held waiting close to the cutting blades for the whole of the time that it takes for the other reel to be unwound, it can happen that very small scraps of web are cut off and, becoming electrostatically charged, are transported together with the cut pieces with disadvantages of various nature.